ASCAObservations of the Symbiotic System CH Cygni

Abstract

The symbiotic system CH Cyg was observed using ASCA on 1994 October. The energy spectrum clearly consists of two different components that are separated at the energy 2 keV. The soft component is represented by a superposition of the two optically thin thermal plasma emissions, the temperatures of which are 0.2 and 0.7 keV. The hard component can also be represented by the optically thin thermal plasma emission but with an order of magnitude higher temperature of 7.3 keV, undergoing a strong photoelectric absorption with NH ~ 1023 cm-2. This hard component shows a time variation as fast as ~100 s. These characteristics of the hard component strongly indicate that CH Cyg system contains an accreting white dwarf. From the temperature of the hard X-ray component, a firm lower limit of the mass of the white dwarf is obtained to be 0.44 M☉. The lower limit can hardly be reconciled with the idea that CH Cyg is the triple system with the inner binary period of 756 days, because the cool giant mass becomes greater than 7.1 M☉, although it is well established to be the SRb variable. If the cool giant mass is 1 M☉, the mass function should be greater than 0.04 M☉ for i = 90°.